At the German Congress for Laboratory Medicine (DKLM) 2025 in Leipzig, a research team presented a study on pre-analytical factors in potassium determination. Potassium is one of the most common laboratory parameters with a narrow physiological range in the body, but shows limited stability in vitro. Factors such as hemolysis, delayed centrifugation, sample material and storage conditions significantly influence the quality of the results. The revision of the 2023 guidelines of the German Medical Association, which prescribes lithium heparin as a material for potassium measurements, has intensified the discussion about optimal samples.

The study by the Institute of Clinical Chemistry and Laboratory Medicine of the University Medical Center Oldenburg and the University Medical Center Greifswald evaluates various pre-analytical influences, in particular sample material and storage temperature. Delayed centrifugation and the influence of platelet count are also taken into account. 120 subjects took part in the study, with data for 30 presented. 29 samples were taken per subject, including pairs of lithium heparin and serum per manufacturer as well as EDTA samples for blood counts. Four manufacturers supplied materials. Samples were centrifuged immediately after collection and stored at room temperature or four degrees Celsius. Measurements were made with ion-selective electrodes on an analyzer.

At baseline zero hours, the mean potassium concentration in lithium heparin samples was 3.87 millimoles per liter, and significantly higher in serum samples at 4.13 millimoles per liter. The mean difference was 0.26 millimoles per litre, with individual variations of minus 0.01 to 0.51 millimoles per litre.

In the case of delayed centrifugation, potassium levels in lithium heparin rose by up to 6.59 percent and in serum by up to 13.08 percent after two hours of storage at four degrees Celsius. After eight hours, the increases reached 36 percent in lithium heparin and 38 percent in serum. At room temperature, values in lithium heparin remained stable for up to four hours, with a slight increase of 0.06 millimoles per liter after six and eight hours. In serum, baseline values were 6.73 percent higher and stabilized up to eight hours.

Platelet counts varied from 151 to 447 per nanoliter and correlated moderately with elevated serum potassium levels with a coefficient of 0.502.

In summary, serum samples showed significantly higher potassium concentrations than lithium heparin, especially when stored at four degrees Celsius. At room temperature, values in both materials stabilize, with the well-known bias for serum. The manufacturer recommends analysis within two hours; at four degrees Celsius, values then increase by 6.6 percent in lithium heparin and 13.1 percent in serum. High platelet counts can cause falsely elevated serum potassium levels, even in the reference range, but the correlation is weak, making platelet-based correction inappropriate. If centrifugation is only carried out after eight hours and stored at four degrees Celsius – necessary for other parameters – potassium results are severely impaired and insufficient for patient care. The presentation urges careful handling in order to avoid misdiagnoses.

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Editor: X-Press Journalistenbüro GbR

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